[InstCombine] improve fold for icmp-ugt-ashr

Existing condition for
fold icmp ugt (ashr X, ShAmtC), C --> icmp ugt X, ((C + 1) << ShAmtC) - 1
missed some boundary. It cause this fold don't work for some cases, and the
reason is due to signed number overflow.

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D127188